Many conventional reservoir rocks have substantial amounts of crude oil in the pore system when recovered. Over time this oil losses its lighter components to evaporation and what remains is called dead oil. For traditional core analysis the dead oil must be removed before proper results can be obtained for porosity, permeability and most other rock properties that involve storage capacity or fluid transport. Removing the dead oil often requires days, weeks, or even months of soaking and flushing with organic solvents. Many of the solvents used are potentially hazardous either from toxicity, flammability, or both. In addition, solvent extraction may remove bitumen which could lead to overestimation of porosity and permeability in organic rich shales.
Digital rock physics (DRP) uses scanning electron microscope (SEM) images to determine many rock properties of interest without requiring the above-mentioned core cleaning process. This is possible, in part, due to the contrast in density and X-ray attenuation between dead oil and rock framework minerals. For example, even for a conventional core sample that has not been cleaned, DRP analysis is able to count dead oil as pore space.
Available geochemistry data shows that there is a substantial ratio of oil to organic matter solids (OMS) or oil to total organic carbon (TOC) in shale samples, perhaps in the range of 20-30%. Conventional core cleaning of these shale samples is problematic due to their low permeability. DRP analysis of the shale samples is also problematic due to the dead oil being essentially indistinguishable from the OMS in SEM images of the shale samples. This is true whether using secondary electron or backscatter electron imaging. To the extent DRP analysis counts dead oil as OMS (or does not count dead oil as pore space), the accuracy of the porosity estimate as well as other rock properties of a shale sample decreases.
It should be understood, however, that the specific embodiments given in the drawings and detailed description below do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and other modifications that are encompassed in the scope of the appended claims.